The average Calgary homeowner may believe the siding they see, is all there is to their waterproofing. However, nothing could be further from the truth, because there is also sheathing under siding. There are tiny gaps between the siding strips, and strong wind could blow moisture and cold air through them.

That’s why home builders and siding specialists add two more layers of protection beneath the siding:

• Waterproof building-paper to catch any moisture and air that penetrates.
• Plywood sheathing to add strength and act as last-resort weatherproofing.

How and Why this Plywood Can Fail

Plywood becomes wet faster, but dries out sooner. However, it can deteriorate if left damp for a long time. The material is therefore unsuitable for exposed exterior use. This is why we lay waterproof building paper over it, to protect sheathing under siding from moisture.

We always remove the building paper along with the siding strips, so we can inspect the sheathing underneath them and replace sections as required. Other contractors often install new paper over existing paper, or simply install new siding over old paper.

This means those contractors cannot inspect the condition of the plywood sheathing. This helps them save money, and quote lower prices or increase their profit. We wrote this post to provide tips to test your plywood sheathing under siding for damp rot, in case you have this problem.

How to Inspect Sheathing under Siding

Rotting plywood can gradually spread, as moisture leaks through failing waterproof building paper. That’s why the wise homeowner inspects their siding regularly, especially on elevations facing directly into stormy weather.

Vulnerable Areas Around the House

Some areas of a home may be more open to moisture intrusion. Here we think of walls below leaky roofs and eavestroughs, and around windows and outside doors. Siding adjoining decks, wet rooms, and basements can also be vulnerable. But by the same token, cracked / broken siding can also be the start of the problem.

Warning Signs on the Siding Itself

Fiber-cement siding may show damp spots after rain. The paint may also start fading, or changing colour. Vinyl siding is not so obliging, and you may need to probe deeper. Try looking inside the house for damp spots or mold on outside walls.

Try Probing the Sheathing Under Siding

Only try this if, and only if you suspect rotting siding on the wall of your house. Obtain a thin, sharp bradawl, and very, very gently insert it between two siding strips at this point. You should feel solid resistance.

If you do not, then you appear to have a problem, because sheathing under siding is not supposed to be soft (or there’s a piece missing). This could be a good time to call Valiant Exteriors at (403)829-1661 and ask for advice. Give us a call when it suits.

More Information

Oriented Strand Board or Plywood

Cedar Shake Life in the Chemistry

Strip Sheathing Before Building Paper and Siding on a Period Barn (Library of Congress)